Hydraulic tappet (hydraulic valve lifter) can be used to automatically compensate for valve lash (valve clearance), to reduce shock and noise of valve train (valve gear), to reduce abrasion or wear of working surfaces of the valve train, and to improve emission and combustion efficiency of thane engine.
The structures of existing hydraulic tappets include several different types as disclosed in the prior art.
As shown in FIG. 1, Chinese patent publication CN201593451U discloses a hydraulic support element, wherein the plunger is machined after cold heading. The machining processes are complicated, and these processes take time and efforts. After the process cutting off material to make the neck portion of the plunger, the metal flow line of the material is cut off. The thickness of the neck is smaller than those of other portions. The sizes of the two cross-sections abruptly change leading to concentration of reactive forces. This greatly impacts the overall strength of the plunger.
As shown in FIG. 2, Chinese patent publication CN201228569Y discloses a hydraulic support element, wherein the plunger comprises a narrow V-shaped circumferential groove on the external diameter, and an oil hole is opened above the V-shaped circumferential groove. A drawback of the V-shaped circumferential groove structure is that when an upper limit clamp spring is assembled, the clamp spring can only slide up and down with the plunger, and may cause fracture on abrasion faces. The position of the oil hole is on an external cylinder surface above the V-shaped groove. When a hydraulic medium passes through the small hole, impurities contained in the hydraulic medium may enter the gap between the housing and the plunger, causing the hydraulic tappet to stuck and lose automatic regulation function.
As shown in FIG. 3, Chinese patent publication CN102767405 discloses a hydraulic support element, wherein the plunger comprises two components. Each single component is easier to machine. However, because two components are machined separately, the production costs are higher. Especially, a variation of the two contact end faces must meet a strict requirement. If the variation is over a tolerance range, a lateral force will be generated, and the plunger may be stuck in the housing, impacting the function of the product.
Chinese patent publication CN102788154 discloses an integral type hydraulic tappet plunger and a method of integral roll forming thereof. A ball portion, a shaft portion and a body portion are formed as a unitary body and have the same wall thickness. Thus, this solves the technical defects of the split type plunger and the traditional machining plunger.
However, after research and investigation, the present inventors found that stresses at different locations on the plunger are not homogeneous. Especially, the head and neck portions are key areas of stress, where lies the hidden danger of stress fatigue. Although the overall strength of the plunger is increased by using the equal wall thickness integral forming process, the technical problems of stress concentration is not solved.